1. Field of the Invention
The present invention relates to a piezoelectric resonator container for containing therein piezoelectric resonator pieces, such as quartz crystal resonators, to a piezoelectric resonator using the piezoelectric resonator container, to a circuit board having the piezoelectric resonator mounted thereon, and to a method for manufacturing a piezoelectric resonator. More particularly, the present invention relates to a technique for sealing an open end of an elliptical or nearly elliptical cylindrical case used in a piezoelectric resonator container.
2. Background of Related Art
Piezoelectric resonators used in various electronic devices are incorporated into the electronic devices in a state of being contained in containers. In some piezoelectric resonators, thin plate piezoelectric resonators are used in which electrodes are formed in predetermined patterns on piezoelectric resonator pieces, such as the tuning fork type or AT cut quartz crystal pieces. Therefore, if elliptical or nearly elliptical cylindrical cases are used corresponding to the shape of the piezoelectric resonator pieces, the piezoelectric resonator pieces can be efficiently contained, and the- thickness of the piezoelectric resonators can be reduced. An example of such containers is disclosed in Japanese Unexamined Utility Model Application Publication No. 2-55727. The piezoelectric resonator container has a structure such that a piezoelectric resonator is contained in a nearly elliptical case, and a stem is pressed into the open end of the case to be sealed.
However, according to the structure in which the nearly elliptical case is sealed by pressure as in the piezoelectric resonator container disclosed in Japanese Unexamined Utility Model Publication No. 2-55727, since the contact surface pressure between the case and the stem varies in a circumferential direction, leakage tends to occur at a portion of low contact surface pressure.
As a piezoelectric resonator container for solving such a problem, Domestic Republication of PCT international Publication for Patent Application No. WO97/08826 discloses a piezoelectric resonator container in which a pressure allowance is maximized between a minor axis portion and a major axis portion, when pressing a stem having an elliptical or nearly elliptical outer peripheral shape into a case having an elliptical or nearly elliptical inner peripheral shape. According to this arrangement, since the pressure allowance of a section where incomplete gas-tightness is empirically most likely to occur is maximized, the incomplete gas-tightness at this portion can be prevented.
However, in the piezoelectric resonator container in which the pressure allowance is maximized between the minor axis portion and the major axis portion, a portion is generated where stress (tensile stress) applied in the circumferential direction of the case during the application of pressure has a maximum. As a result, the case may break at this portion, and incomplete gas-tightness occurs.
It is an object of the present invention to realize a piezoelectric resonator container which solves the above problems and. in which incomplete gas-tightness does not occur even if an elliptical or nearly elliptical case is used, a piezoelectric resonator using the same, a circuit board having the piezoelectric resonator mounted thereon, and a method for manufacturing a piezoelectric resonator, and to realize a piezoelectric resonator container in which a piezoelectric resonator piece does not come into contact with the inner periphery of the case even if an oblate case, such as an elliptical or nearly elliptical case, is used.
In accordance with one aspect of the invention, there is provided a piezoelectric resonator container which may a consist of a cylindrical case, one end of which is an open end; and a stem for sealing the open end of the case, wherein, in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem includes a minor axis portion having a first radius of curvature, and a major axis portion having a second radius of curvature smaller than the first radius of curvature, and a tangent to the first radius of curvature coincides with a tangent to the second radius of curvature at a connection portion of the first radius of curvature and the second radius of curvature, wherein, in a cross section of the case in a direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case includes a minor axis portion having a third radius of curvature, and a major axis portion having a fourth radius of curvature smaller than the third radius of curvature, and a tangent to the third radius of curvature coincides with a tangent to the fourth radius of curvature at a connection portion of the third radius of curvature and the fourth radius of curvature, and wherein a pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis of the case toward the minor axis.
In accordance with another aspect of the invention, there is provided a piezoelectric resonator in which a piezoelectric resonator piece is contained in a piezoelectric resonator container having a cylindrical case, one end of which is an open end, and a stem for sealing the open end of the case, wherein, in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem includes a minor axis portion having a first radius of curvature, and a major axis portion having a second radius of curvature smaller than the first radius of curvature, and a tangent to the first radius of curvature coincides with a tangent to the second radius of curvature at a connection portion of the first radius of curvature and the second radius of curvature, wherein, in a cross section of the case in a direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case includes a minor axis portion having a third radius of curvature, and a major axis portion having a fourth radius of curvature smaller than the third radius of curvature, and a tangent to the third radius of curvature coincides with a tangent to the fourth radius of curvature at a connection portion of the third radius of curvature and the fourth radius of curvature, and wherein a pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis of the case toward the minor axis.
In the above aspect of the invention, since the outer periphery of the stem and the inner periphery of the case have a nearly elliptical shape formed of two types of large and small arcs, a flat piezoelectric resonator piece can be efficiently contained, and the thickness of the piezoelectric resonator can be reduced. Since the pressure allowance is increased at the minor axis portion where the vertical stress tends to decrease, the vertical stress is above the lower limit value over the entirety thereof. Therefore, incomplete gas-tightness due to inadequate sealing does not occur. Furthermore, since the pressure allowance is set to be small at the major axis portion where the tensile stress tends to increase, the tensile stress is below the upper limit value over the entirety thereof. For this reason, incomplete gas-tightness due to the breaking of the case does not occur. Therefore, according to the present invention, a piezoelectric resonator with a high degree of reliability can be constructed.
In accordance with an aspect of the invention, there is provided a piezoelectric resonator container which may consist of a cylindrical case, one end of which is an open end; and a stem for sealing the open end of the case, wherein, in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem has an elliptical shape, wherein, in a cross section of the case in a direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case has an elliptical shape, and wherein a pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis of the case toward the minor axis.
In accordance with another aspect of the invention, there is provided a piezoelectric resonator in which a piezoelectric resonator piece is contained in a piezoelectric resonator container having a cylindrical case, one end of which is an open end, and a stem for sealing the open end of the case, wherein, in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem has an elliptical shape, wherein, in a cross section of the case in a direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case has an elliptical shape, and wherein a pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis of the case toward the minor axis.
In the above aspect of the invention, since the outer periphery of the stem and the inner periphery of the case have an elliptical shape, a flat piezoelectric resonator piece can be efficiently contained, and the thickness of the piezoelectric resonator can be reduced. Since the pressure allowance is set to be large at the minor axis portion where the vertical stress tends to decrease, the vertical stress is above the lower limit value over the entirety thereof. In addition, since the pressure allowance is set to be small at the major axis portion where the tensile stress tends to increase, the tensile stress is below the upper limit value over the entirety thereof. Therefore, since incomplete gas-tightness due to inadequate sealing or the breaking of the case does not occur, a piezoelectric resonator with a high degree of reliability can be constructed.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the inner peripheral edge of the open end of the case is formed into a tapered face in a cross section in the axial direction of the case, and wherein a shoulder part located in the pressure direction of the stem is arcuate in a cross section in the axial direction of the case, and the radius of curvature of the arc is larger at the minor axis portion than that at the major axis portion of the stem.
In the above aspect of the invention, since the pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis to the minor axis, only a shoulder part corresponding to the minor axis portion of the stem abuts against the tapered face of the open end of the case when the stem is registered against the open end of the case. Therefore, in the above aspect of the invention, a problem of the occurrence of displacement of the stem is not solved.
In the above aspect of the invention, however, since the radius of curvature of the arc of the shoulder part of the stem is small at the major axis portion and is large at the minor axis portion, the shoulder part tends to project at the major axis portion. Therefore, since the shoulder part of the minor axis portion of the stem, the shoulder part between the minor axis portion and the major axis portion, and the shoulder part of the major axis portion abut against the tapered face of the case at about the same time, the shoulder parts of the stem come into contact with the tapered portion of the case over the entirety thereof. Thus, since the stem is not displaced from the case, incomplete gas-tightness due to the displacement does not occur.
In accordance with another aspect of the invention, there is provided the feature the above aspect, wherein the inner peripheral edge of the open end of the case is formed into a tapered face in a cross section in the axial direction of the case, and wherein a shoulder part located in the pressure direction of the stem is arcuate in a cross section in the axial direction of the case, and the center position of the arc is located toward the major axis portion from the minor axis portion of the stem in the pressure direction.
In the above aspect of the invention, since the center position of the arc formed on the shoulder part of the stem is located toward the major axis portion from the minor axis portion of the stem in the pressure direction, the shoulder part tends to project at the major axis portion. Therefore, the shoulder part of the minor axis portion of the stem, the shoulder part between the minor axis portion and the major axis portion, and the shoulder part of the major axis portion abut against the tapered face of the case at about the same time. Therefore, the shoulder parts of the stem come into contact with the tapered portion of the case over the entirety thereof. Thus, since the stem is not displaced from the case, incomplete gas-tightness due to the displacement does not occur.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the inner peripheral edge of the open end of the case is formed into a tapered face in a cross section in the axial direction of the case, and an angle which the tapered face forms with the axial direction is smaller at the minor axis portion than that at the major axis portion.
In the above aspect of the invention, the shoulder part of the minor axis portion of the stem, the shoulder part between the minor axis portion and the major axis portion, and the shoulder part of the major axis portion abut against the tapered face of the case at about the same time. Therefore, the shoulder parts of the stem come into contact with the tapered portion of the case over the entirety thereof. Thus, since the stem is not displaced from the case, incomplete gas-tightness due to the displacement does not occur.
In accordance with another aspect of the invention, there is provided the feature of the above aspects, wherein the inner peripheral edge of the open end of the case is formed into a tapered face in a cross section in the axial direction of the case, and the position of the tapered face in the axial direction is located toward the minor axis portion from the major axis portion of the case in the pressure direction.
In the above aspect of the invention, the shoulder part of the minor axis portion of the stem, the shoulder part between the minor axis portion and the major axis portion, and the shoulder part of the major axis portion abut against the tapered face of the case at about the same time. Therefore, the shoulder parts of the stem come into contact with the tapered portion of the case over the entirety thereof. Thus, since the stem is not displaced from the case, incomplete gas-tightness due to the displacement does not occur.
In accordance with another aspect of the invention, there is provided the feature of the another aspect, wherein projections abutting against the inner peripheral edge of the open end of the case which is formed as the tapered face are formed at four sections corresponding to between the major axis portion and the minor axis portion of shoulder parts located in the pressure direction.
In the above aspect of the invention, when the stem is registered at the open end of the case, four projections formed between the major axis portion and the minor axis portion abut against the tapered face of the case. In this state, since the four projections prevent the stem from being rotated with respect to the case, the stem is not displaced from the case. Therefore, incomplete gas-tightness due to the displacement does not occur.
In accordance with another aspect of the invention, there is provided the feature of, wherein the projections are formed by deep-drawing a metal outer ring constituting the stem.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the projections are formed by solder balls.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the projections are formed by thick solder plating.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the thickness of the case at the major axis portion is less than the thickness at the minor axis portion.
In the above aspect of the invention, since the thickness is reduced at the major axis portion where the tensile stress tends to increase, tensile stress generated at the major axis portion decreases, and the case can be prevented from being broken at the portion. A reduction in the thickness of the case at the minor axis portion offers an advantage in that the case is difficult to deform, even if pressure is applied to the minor axis portion, as compared with a situation in which the case has a uniform thickness.
In accordance with another aspect of the invention, there is provided the feature of any one of the above aspect, wherein, in leads fixed to the stem, diameters of inner lead portions to be superimposed on mounting portions of the piezoelectric resonator in the case are smaller than those of other lead portions.
In the above aspect of the invention, even if the case is flattened and the pressure allowance of the minor axis portion is increased, a gap size between the piezoelectric resonator piece and the inner periphery of the case is wide by the degree of thinness of the inner lead portions. Therefore, since the piezoelectric resonator piece does not touch the inner periphery of the case, even if it is flexed by a fall or the like, breakage or chipping of the piezoelectric resonator piece can be prevented.
In accordance with another aspect of the invention of claim 24, there is provided the feature of any one of the above aspect, in leads fixed to the stem, inner lead portions to be superimposed on mounting portions of the piezoelectric resonator in the case are flattened.
In the above aspect of the invention, even if the case is flattened and the pressure allowance of the minor axis portion is increased, a gap size between the piezoelectric resonator piece and the inner periphery of the case is wide by the degree of flatness of the inner lead portions. Therefore, since the piezoelectric resonator piece does not touch the inner periphery of the case even if it is flexed by a fall or the like, breakage or chipping of the piezoelectric resonator piece can be prevented.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the inner lead portions are formed with projections projecting toward the outside from the flat faces of the inner lead portions.
In the above aspect of the invention, the inner lead portions can be pressed with uniform force toward the piezoelectric resonator piece via the projections, even if the flat inner lead portions are inclined or rotated. Therefore, the inner lead portions and mounting portions of the piezoelectric resonator piece can be securely connected by a conductive adhesive agent or solder.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the projections are formed by solder balls.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the projections are formed by thick solder plating.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the projections are formed by deformed portions of the inner lead portions.
In accordance with another aspect of the invention, there is provided the feature of any one of the above aspect, wherein, in leads fixed to the stem, inner lead portions to be superimposed on mounting portions of the piezoelectric resonator piece in the case are bent so that an end portion of the piezoelectric resonator piece is placed in the center of a thickness direction of the piezoelectric resonator.
In the above aspect of the invention, even if the case is flattened and the pressure allowance of the minor axis portion is increased, a gap size between the piezoelectric resonator piece and the inner periphery of the case is wide by the degree of bending of the inner lead portions. Therefore, since the piezoelectric resonator piece does not touch the inner periphery of the case, even if it is flexed by a fall or the like, breakage or chipping of the piezoelectric resonator piece can be prevented.
In accordance with another aspect of the invention, there is provided the feature of any one of the above aspects, wherein the inner lead portions are bent so that the end portion of the piezoelectric resonator piece is placed in the center of the thickness direction of the piezoelectric resonator.
In the above aspect of the invention, even if the case is flattened and the pressure allowance of the minor axis portion is increased, a gap size between the piezoelectric resonator piece and the inner periphery of the case is wide by the degree of bending of the inner lead portions. Therefore, since the piezoelectric resonator piece does not touch the inner periphery of the case even if it is flexed by a fall or the like, breakage or chipping of the piezoelectric resonator piece can be prevented.
In accordance with another aspect of the invention, there is provided a method for manufacturing a piezoelectric resonator in which a piezoelectric resonator piece is contained in a piezoelectric resonator container using a cylindrical case, one end of which is an open end, and a stem for sealing the open end of the case, wherein, in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem includes a minor axis portion having a first radius of curvature, and a major axis portion having a second radius of curvature smaller than the first radius of curvature, and a tangent to the first radius of curvature coincides with a tangent to the second radius of curvature at a connection portion of the first radius of curvature and the second radius of curvature, wherein, in a cross section of the case in an direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case includes a minor axis portion having a third radius of curvature, and a major axis portion having a fourth radius of curvature smaller than the third radius of curvature, and a tangent to the third radius of curvature coincides with a tangent to the fourth radius of curvature at a connection portion of the third radius of curvature and the fourth radius of curvature, wherein a pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis of the case toward the minor axis, wherein projections abutting against the inner peripheral edge of the open end of the case which is formed as the tapered face are formed at four sections corresponding to between the major axis portion and the minor axis portion of shoulder parts located in the pressure direction, and wherein, when pressing the stem into the open end of the case, the case and the stem are relatively moved by vibrations to thereby determine positioning of the case and the stem.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the vibrations are generated by an ultrasonic vibrator.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the vibrations are generated by a piezoelectric element.
In accordance with another aspect of the invention, there is provided a method for manufacturing a piezoelectric resonator in which a piezoelectric resonator piece is contained in a piezoelectric resonator container using a cylindrical case, one end of which is an open end, and a stem for sealing the open end of the case, wherein, in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem includes a minor axis portion having a first radius of curvature, and a major axis portion having a second radius of curvature smaller than the first radius of curvature, and a tangent to the first radius of curvature coincides with a tangent to the second radius of curvature at a connection portion of the first radius of curvature and the second radius of curvature, wherein, in a cross section of the case in a direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case includes a minor axis portion having a third radius of curvature, and a major axis portion having a fourth radius of curvature smaller than the third radius of curvature, and a tangent to the third radius of curvature coincides with a tangent to the fourth radius of curvature at a connection portion of the third radius of curvature and the fourth radius of curvature, and wherein a pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis of the case toward the minor axis.
In accordance with another aspect of the invention, there is provided a method for manufacturing a piezoelectric resonator in which a piezoelectric resonator piece is contained in a piezoelectric resonator container using a cylindrical case, one end of which is an open end, and a stem for sealing the open end of the case, wherein, in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem has an elliptical shape, wherein, in a cross section of the case in a direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case has an elliptical shape, and wherein a pressure allowance when pressing the stem into the open end of the case monotonically increase from the major axis of the case toward the minor axis.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the inner peripheral edge, of the open end of the case is formed into a tapered face in a cross section in the axial direction of the case, and wherein a shoulder part located in a direction of the pressing of the stem is arcuate in a cross section in the axial direction of the case, and the radius of curvature of the arc is larger at the minor axis portion than that at the major axis portion of the stem.
In accordance with another aspect of the invention, there is provided the aspect of the invention of the above aspect, wherein the inner peripheral edge of the open end of the case is formed into a tapered face in a cross section in the axial direction of the case, and wherein a shoulder part located in a pressure direction of the stem is arcuate in a cross section in the axial direction of the case, and the center position of the arc is located toward the major axis portion from the minor axis portion of the stem in the pressure direction.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the inner peripheral edge of the open end of the case is formed into a tapered face in a cross section in the axial direction of the case, and an angle which the tapered face forms with the axial direction is smaller at the minor axis portion than that at the major axis portion.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the inner peripheral edge of the open end of the case is formed into a tapered face in a cross section in the axial direction of the case, and the position of the tapered face in the axial direction is located toward the minor axis portion from the major axis portion of the case in the pressure direction.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein projections abutting against the inner peripheral edge of the open end of the case which is formed as the tapered face are formed at four sections corresponding to between the major axis portion and the minor axis portion of shoulder parts located in the pressure direction.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the projections are formed by deep-drawing a metal outer ring constituting the stem.
In accordance with another aspect of the invention, there is provided the feature of the above aspects, wherein the projections are formed by solder balls.
In accordance with another aspect of the invention, there is provided the feature of the above aspects, wherein the projections are formed by thick solder plating.
In accordance with another aspect of the invention, there is provided the feature of the above aspects, wherein the thickness of the case at the major axis portion is less than the thickness at the minor axis portion.
In accordance with another aspect of the invention, there is provided the feature of any one of the above aspects, wherein, in leads fixed to the stem, inner lead portions to be superimposed on mounting portions of the piezoelectric resonator piece in the case are bent so that an end portion of the piezoelectric resonator piece is placed in the center of a thickness direction of the piezoelectric resonator.
In accordance with another aspect of the invention, there is provided the feature of any one of the above aspects, wherein, in leads fixed to the stem, diameters of inner lead portions to be superimposed on mounting portions of the piezoelectric resonator in the case are smaller than those of other lead portions.
In accordance with another aspect of the invention, there is provided the feature of any one of the above aspects, wherein, in leads fixed to the stem, inner lead portions to be superimposed on mounting portions of the piezoelectric resonator in the case are flattened.
In accordance with another aspect of the invention, there is provided the feature of the above aspects, wherein the inner lead portions are formed with projections projecting toward the outside from the flat faces of the inner lead portions.
In accordance with another aspect of the invention, there is provided the feature of the above aspects, wherein the projections are formed by solder balls.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the projections are formed by thick solder plating.
In accordance with another aspect of the invention, there is provided the feature of the above aspect, wherein the projections are formed by deformed portions of the inner lead portions.
In accordance with another aspect of the invention, there is provided the feature of any one of the above aspect, wherein the inner lead portions are bent so that an end portion of the piezoelectric resonator piece is placed in the center of a thickness direction of the piezoelectric resonator.
In accordance with another aspect of the invention, there is provided the feature of the above aspects, wherein the piezoelectric resonator piece is mounted on the inner leads that are bent in advance so that the end portion of the piezoelectric resonator piece is placed in the center of the thickness direction of the piezoelectric resonator.
In accordance with another aspect of the invention, there is provided the feature of the above aspects, wherein the piezoelectric resonator piece is mounted on the inner leads while bending the inner leads so that the end portion of the piezoelectric resonator piece is placed in the center of the thickness direction of the piezoelectric resonator.
In accordance with another aspect of the invention, there is provided a circuit board having mounted thereon a piezoelectric resonator in which a piezoelectric resonator piece is contained in a piezoelectric resonator container having a cylindrical case, one end of which is an open end, and a stem for sealing the open end of the case, wherein in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem includes a minor axis portion having a first radius of curvature, and a major axis portion having a second radius of curvature smaller than the first radius of curvature, and a tangent to the first radius of curvature coincides with a tangent to the second radius of curvature at a connection portion of the first radius of curvature and the second radius of curvature, wherein, in a cross section of the case in a direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case includes a minor axis portion having a third radius of curvature, and a major axis portion having a fourth radius of curvature smaller than the third radius of curvature, and a tangent to the third radius of curvature coincides with a tangent to the fourth radius of curvature at a connection portion of the third radius of curvature and the fourth radius of curvature, and wherein a pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis of the case toward the minor axis.
In the above aspect of the invention, since the outer periphery of the stem and the inner periphery of the case have a nearly elliptical shape formed of two type of large and small arcs, a flat piezoelectric resonator piece can be efficiently contained, and the thickness of the piezoelectric resonator can be reduced. Thus, if the piezoelectric resonator is mounted on a circuit board, the thickness of the circuit board can be reduced. Since the pressure allowance is increased at the minor axis portion where the vertical stress tends to decrease, the vertical stress is above the lower limit value over the entirety thereof. Therefore, incomplete gas-tightness due to inadequate sealing does not occur. Furthermore, since the pressure allowance is set to be small at the major axis portion where the tensile stress tends to increase, the tensile stress is below the upper limit value over the entirety thereof. For this reason, incomplete gas-tightness due to the breaking of the case does not occur. Therefore, according to the present invention, a piezoelectric resonator with a high degree of reliability, and a circuit board having the piezoelectric resonator mounted thereon can be constructed.
In accordance with another aspect of the invention, there is provided a circuit board having mounted thereon a piezoelectric resonator in which a piezoelectric resonator piece is contained in a piezoelectric resonator container having a cylindrical case, one end of which is an open end, and a stem for sealing the open end of the case, wherein, in a cross section of the stem in a direction perpendicular to an axial direction of the case at a portion contacting the case, an outer periphery of the stem has an elliptical shape, wherein, in a cross section of the case in a direction perpendicular to an axial direction of the case at a portion contacting the stem, an inner periphery of the case has an elliptical shape, and wherein a pressure allowance when pressing the stem into the open end of the case monotonically increases from the major axis of the case toward the minor axis.
In the above aspect of the invention, since the outer periphery of the stem and the inner periphery of the case have an elliptical shape, a flat piezoelectric resonator piece can be efficiently contained, and the thickness of the piezoelectric resonator can be reduced. Thus, if the piezoelectric resonator is mounted on a circuit board, the thickness of the circuit board can be reduced. Since the pressure allowance is set to be large at the minor axis portion where the vertical stress tends to decrease, the vertical stress is above the lower limit value over the entirety thereof. In addition, since the pressure allowance is set to be small at the major axis portion where the tensile stress tends to increase, the tensile stress is below the upper limit value over the entirety thereof. Therefore, since incomplete gas-tightness due to inadequate sealing or the breaking of the case does not occur, a piezoelectric resonator with a high degree of reliability, and a circuit board having the piezoelectric resonator mounted thereon can be constructed.